Method and apparatus for receiving ACK/NACK in wireless communication system

ABSTRACT

Provided are a method and an apparatus for a user equipment, which is allocated a plurality of serving cells, receiving acknowledgement/negative acknowledgement (ACK/NACK) in a wireless communication system. The method comprises: transmitting uplink data through a physical uplink shared channel (PUSCH); and receiving ACK/NACK with respect to the uplink through a physical hybrid-ARQ indicator channel (PHICH), wherein a serving cell that receives the ACK/NACK is selected from one or more serving cells, which the user equipment monitors to detect an uplink grant that schedules the PUSCH.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/350,551, filed on Apr. 8, 2014, now U.S. Pat. No. 9,843,430, which isthe National Stage filing under 35 U.S.C. 371 of InternationalApplication No. PCT/KR2012/009142, filed on Nov. 1, 2012, which claimsthe benefit of U.S. Provisional Application Nos. 61/650,989, filed onMay 23, 2012, 61/594,387, filed on Feb. 3, 2012, and 61/554,470, filedon Nov. 1, 2011, the contents of which are all hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method and apparatus for receivingacknowledgement/not-acknowledgement (ACK/NACK) indicating receptionacknowledgement in a wireless communication system.

Related Art

One of the most important requirements of a next generation wirelesscommunication system is to support a high data rate. For this, varioustechniques such as multiple input multiple output (MIMO), cooperativemultiple point transmission (CoMP), relay, etc., have been underresearch, but the most fundamental and reliable solution is to increasea bandwidth.

However, a frequency resource is in a saturation state at present, andvarious techniques are partially used in a wide frequency band. For thisreason, as a method of ensuring a broadband bandwidth for satisfying arequired higher data rate, a carrier aggregation (CA) is introduced. Inconcept, the CA is designed such that a basic requirement which allowsseparate bands to operate respective independent systems, and aplurality of bands are aggregated into one system. In this case, a bandthat can be independently managed is defined as a component carrier(CC).

The latest communication standard (e.g., 3GPP LTE-A or 802.16m)considers to expand its bandwidth to 20 MHz or higher. In this case, awideband is supported by aggregating one or more CCs. For example, ifone CC corresponds to a bandwidth of 5 MHz, four carriers are aggregatedto support a bandwidth of up to 20 MHz. As such, a system supportingcarrier aggregation is called a carrier aggregation system.

Meanwhile, a wireless communication system considers a system in which abase station supports a greater number of user equipments in comparisonwith the legacy system. For example, one base station may have tosupport the greater number of user equipments when a technique such asmachine type communication (MTC), enhanced multi user multi input multioutput (MIMO), etc., is applied.

In this case, it may be difficult to transmit control information to aplurality of user equipments when using only a physical downlink controlchannel (PDCCH) in long term evolution (LTE)) conventionally used totransmit the control information. This is because there may be a problemin that a radio resource of the PDCCH is insufficient or an interferencebecomes serious. In order to solve such a problem, it is considered toallocate a new control region to a radio resource region in which datais transmitted in the legacy system. Such a new control channel iscalled an enhanced-PDCCH (E-PDCCH).

Meanwhile, a base station transmits acknowledgement/not-acknowledgement(ACK/NACK) for uplink data received from a user equipment through aphysical hybrid-ARQ indicator channel (PHICH). The PHICH is located in aregion to which a conventional control channel, i.e., PDCCH, isallocated. The PHICH may also have a radio resource shortage problem oran interference problem when the number of user equipments supported bythe base station is increased and a carrier aggregation is supported.Therefore, it is considered to introduce a new channel for ACK/NACKtransmission, and such a channel is called an enhanced-PHICH (E-PHICH).

However, even if a wireless communication system supports the E-PDCCHand the E-PHICH, the E-PDCCH and the E-PHICH may not be included in alluser equipments, all carriers, and all subframes. That is, the E-PDCCHand the E-PHICH may be selectively used. In addition, when the carrieraggregation is supported, multiple carriers may be allocated to aspecific user equipment.

In this case, it is necessary to regulate which carrier and whichchannel will be used by the user equipment to receive ACK/NACK. That is,in which way the base station will transmit ACK/NACK and how the userequipment will receive the ACK/NACK may become a matter to be discussed.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for receivingacknowledgement/not-acknowledgement (ACK/NACK) in a wirelesscommunication system.

According to an aspect of the present invention, a method of receivingacknowledgement/not-acknowledgement (ACK/NACK) of a user equipment towhich a plurality of serving cells are allocated in a wirelesscommunication system is provided. The method includes: transmittinguplink data through a physical uplink shared channel (PUSCH); andreceiving ACK/NACK for the uplink data through a physical hybrid-ARQindicator channel (PHICH), wherein a serving cell that receives theACK/NACK is selected from one or more serving cells to be monitored bythe user equipment to detect an uplink grant that schedules the PUSCH.

According to another aspect of the present invention, a method ofreceiving ACK/NACK of a user equipment to which a plurality of servingcells are allocated in a wireless communication system is provided. Themethod includes: receiving PHICH cell indicator information indicating aPHICH cell; transmitting uplink data through a PUSCH; and receivingACK/NACK for the uplink data through a PHICH, wherein the ACK/NACK isreceived through a serving cell indicated by the PHICH cell indicatorinformation.

According to another aspect of the present invention, a method ofreceiving ACK/NACK of a user equipment to which a plurality of servingcells are allocated in a wireless communication system is provided. Themethod includes: transmitting uplink data through a PUSCH; receiving anuplink grant through an enhanced-physical downlink control channel(E-PDCCH); and retransmitting the uplink data or transmitting new uplinkdata on the basis of the uplink grant, wherein the uplink grant isincluded in a subframe in which the uplink grant is received, instead ofa PHICH which transmits ACK/NACK for the uplink data.

According to another aspect of the present invention, a method ofreceiving ACK/NACK of a user equipment to which a plurality of servingcells are allocated in a wireless communication system is provided. Themethod includes: transmitting uplink data through a PUSCH; and receivingACK/NACK for the uplink data through a PHICH, wherein a serving cell forreceiving the ACK/NACK is a serving cell that receives an uplink grantfor scheduling the PUCSH, the uplink grant is received through anE-PDCCH, and the E-PDCCH is a control channel which is decoded by usinga reference signal that is specific to the user equipment.

According to another aspect of the present invention, there is provideda user equipment including: a radio frequency (RF) unit for transmittingand receiving a radio signal; and a processor operatively coupled to theRF unit, wherein the processor is configured for: transmitting uplinkdata through a PUSCH; and receiving ACK/NACK for the uplink data througha PHICH, wherein a serving cell that receives the ACK/NACK is selectedfrom one or more serving cells to be monitored by the user equipment todetect an uplink grant that schedules the PUSCH.

According to another aspect of the present invention, there is provideda user equipment including: an RF unit for transmitting and receiving aradio signal; and a processor operatively coupled to the RF unit,wherein the processor is configured for: receiving PHICH cell indicatorinformation indicating a PHICH cell; transmitting uplink data through aPUSCH; and receiving ACK/NACK for the uplink data through a PHICH,wherein the ACK/NACK is received through a serving cell indicated by thePHICH cell indicator information.

According to another aspect of the present invention, there is provideda user equipment including: an RF unit for transmitting and receiving aradio signal; and a processor operatively coupled to the RF unit,wherein the processor is configured for: transmitting uplink datathrough a PUSCH; receiving an uplink grant through an E-PDCCH; andretransmitting the uplink data or transmitting new uplink data on thebasis of the uplink grant, wherein the uplink grant is included in asubframe in which the uplink grant is received, instead of a PHICH whichtransmits ACK/NACK for the uplink data.

According to another aspect of the present invention, there is provideda user equipment including: an RF unit for transmitting and receiving aradio signal; and a processor operatively coupled to the RF unit,wherein the processor is configured for: transmitting uplink datathrough a PUSCH; and receiving ACK/NACK for the uplink data through aPHICH, wherein a serving cell for receiving the ACK/NACK is a servingcell that receives an uplink grant for scheduling the PUCSH, the uplinkgrant is received through an E-PDCCH, and the E-PDCCH is a controlchannel which is decoded by using a reference signal that is specific tothe user equipment.

According to the present invention, a user equipment can receiveacknowledgement/not-acknowledgement (ACK/NACK) without an ambiguity in awireless communication system in which an additional control channel,i.e., an enhanced-physical downlink control channel (E-PDCCH), isconfigured in addition to the conventional physical downlink controlchannel (PDCCH).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a downlink radio frame in 3^(rd) generationpartnership project (3GPP) long term evolution-advanced (LTE-A).

FIG. 2 shows an example of a resource grid for one downlink (DL) slot.

FIG. 3 is a block diagram showing a structure of a physical downlinkcontrol channel (PDCCH).

FIG. 4 shows an example of monitoring a PDCCH.

FIG. 5 shows a structure of an uplink (UL) subframe.

FIG. 6 shows UL synchronous hybrid automatic repeat request (HARQ) in3GPP LTE.

FIG. 7 shows a structure of a physical hybrid-ARQ indicator channel(PHICH) in 3GPP LTE.

FIG. 8 shows an example of comparing a legacy single-carrier system anda carrier aggregation system.

FIG. 9 shows an example of cross-carrier scheduling in a carrieraggregation system.

FIG. 10 shows an example of scheduling when cross-carrier scheduling isconfigured in a carrier aggregation system.

FIG. 11 shows an example of configuring an enhanced-PHICH (E-PHICH)region and an enhanced-PDCCH (E-PDCCH) region.

FIG. 12 shows an example of allocating an E-PHICH in a search space.

FIG. 13 shows a method of receiving acknowledgement/not-acknowledgement(ACK/NACK) according to the embodiment 2-2.

FIG. 14 shows an example of a method of configuring a PHICH cell andreceiving ACK/NACK.

FIG. 15 shows an example of a method of receiving ACK/NACK of a userequipment (UE).

FIG. 16 shows a method of receiving ACK/NACK of a UE according to theembodiment 4-2.

FIG. 17 shows a structure of a base station (BS) and a UE according toan embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Long term evolution (LTE) of the 3^(rd) generation partnership project(3GPP) standard organization is a part of an evolved-universal mobiletelecommunications system (E-UMTS) using an evolved-universalterrestrial radio access network (E-UTRAN). The LTE employs anorthogonal frequency division multiple access (OFDMA) in a downlink andemploys single carrier-frequency division multiple access (SC-FDMA) inan uplink. LTE-advance (LTE-A) is an evolution of the 3GPP LTE. Forclarity, the following description will focus on the 3GPP LTE/LTE-A.However, technical features of the present invention are not limitedthereto.

A wireless device may be fixed or mobile, and may be referred to asanother terminology, such as a user equipment (UE), a mobile station(MS), a mobile terminal (MT), a user terminal (UT), a subscriber station(SS), a wireless device, a personal digital assistant (PDA), a wirelessmodem, a handheld device, etc. The wireless device may also be a devicesupporting only data communication such as a machine-type communication(MTC) device.

A base station (BS) is generally a fixed station that communicates withthe wireless device, and may be referred to as another terminology, suchas an evolved-NodeB (eNB), a base transceiver system (BTS), an accesspoint, etc.

Hereinafter, it is described that the present invention is appliedaccording to a 3^(rd) generation partnership project (3GPP) long termevolution (LTE) based on 3GPP technical specification (TS) release 8 or3GPP LTE-advanced (LTE-A) based on 3GPP TS release 10. However, this isfor exemplary purposes only, and thus the present invention is alsoapplicable to various wireless communication networks. In the followingdescription, LTE and/or LTE-A are collectively referred to as LTE.

The wireless device may be served by a plurality of serving cells. Eachserving cell may be defined with a downlink (DL) component carrier (CC)or a pair of a DL CC and an uplink (UL) CC.

The serving cell may be classified into a primary cell and a secondarycell. The primary cell operates at a primary frequency, and is a celldesignated as the primary cell when an initial network entry process isperformed or when a network re-entry process starts or in a handoverprocess. The primary cell is also called a reference cell. The secondarycell operates at a secondary frequency. The secondary cell may beconfigured after an RRC connection is established, and may be used toprovide an additional radio resource. At least one primary cell isconfigured always. The secondary cell may be added/modified/released byusing higher-layer signaling (e.g., a radio resource control (RRC)message).

A cell index (CI) of the primary cell may be fixed. For example, alowest CI may be designated as a CI of the primary cell. It is assumedhereinafter that the CI of the primary cell is 0 and a CI of thesecondary cell is allocated sequentially starting from 1.

FIG. 1 shows a structure of a downlink radio frame in 3GPP LTE-A. Thesection 6 of 3GPP TS 36.211 V10.2.0 (2011-06) “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 10)” may be incorporated herein by reference.

A radio frame includes 10 subframes indexed with 0 to 9. One subframeincludes 2 consecutive slots. A time required for transmitting onesubframe is defined as a transmission time interval (TTI). For example,one subframe may have a length of 1 millisecond (ms), and one slot mayhave a length of 0.5 ms.

One slot may include a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain Since the 3GPP LTE usesorthogonal frequency division multiple access (OFDMA) in a downlink(DL), the OFDM symbol is only for expressing one symbol period in thetime domain, and there is no limitation in a multiple access scheme orterminologies. For example, the OFDM symbol may also be referred to asanother terminology such as a single carrier frequency division multipleaccess (SC-FDMA) symbol, a symbol period, etc.

Although it is described that one slot includes 7 OFDM symbols forexample, the number of OFDM symbols included in one slot may varydepending on a length of a cyclic prefix (CP). According to 3GPP TS36.211 V10.2.0, in case of a normal CP, one slot includes 7 OFDMsymbols, and in case of an extended CP, one slot includes 6 OFDMsymbols.

A resource block (RB) is a resource allocation unit, and includes aplurality of subcarriers in one slot. For example, if one slot includes7 OFDM symbols in a time domain and the RB includes 12 subcarriers in afrequency domain, one RB can include 7×12 resource elements (REs).

FIG. 2 shows an example of a resource grid for one downlink (DL) slot.

Referring to FIG. 2, the DL slot includes a plurality of OFDM symbols ina time domain, and includes N_(RB) resource blocks (RBs) in a frequencydomain. The RB includes one slot in the time domain in a unit ofresource allocation, and includes a plurality of consecutive subcarriersin the frequency domain. The number N_(RB) of RBs included in the DLslot depends on a DL transmission bandwidth configured in a cell. Forexample, in the LTE system, N_(RB) may be any one value in the range of6 to 110. A structure of a UL slot may be the same as the aforementionedstructure of the DL slot.

Each element on the resource grid is referred to as a resource element(RE). The RE on the resource grid can be identified by an index pair(k,l) within the slot. Herein, k(k=0, . . . , N_(RB)×12−1) denotes asubcarrier index in the frequency domain, and l(l=0, . . . , 6) denotesan OFDM symbol index in the time domain.

Although it is described in FIG. 2 that one RB consists of 7 OFDMsymbols in the time domain and 12 subcarriers in the frequency domainand thus includes 7×12 REs, this is for exemplary purposes only.Therefore, the number of OFDM symbols and the number of subcarriers inthe RB are not limited thereto. The number of OFDM symbols and thenumber of subcarriers may change variously depending on a CP length, afrequency spacing, etc. The number of subcarriers in one OFDM symbol maybe any one value selected from 128, 256, 512, 1024, 1536, and 2048.

A DL subframe is divided into a control region and a data region in thetime domain. The control region includes up to first four OFDM symbolsof a first slot in the subframe. However, the number of OFDM symbolsincluded in the control region may vary. A physical downlink controlchannel (PDCCH) and other control channels are allocated to the controlregion, and a physical downlink shared channel (PDSCH) is allocated tothe data region.

As disclosed in 3GPP TS 36.211 V10.2.0, examples of a physical controlchannel in 3GPP LTE/LTE-A include a physical downlink control channel(PDCCH), a physical control format indicator channel (PCFICH), and aphysical hybrid-ARQ indicator channel (PHICH). The PCFICH transmitted ina first OFDM symbol of the subframe carries a control format indicator(CFI) regarding the number of OFDM symbols (i.e., a size of the controlregion) used for transmission of control channels in the subframe. Awireless device first receives the CFI on the PCFICH, and thereaftermonitors the PDCCH.

Unlike the PDCCH, the PCFICH does not use blind decoding, and istransmitted by using a fixed PCFICH resource of the subframe.

The PHICH carries a positive-acknowledgement(ACK)/negative-acknowledgement (NACK) signal for an uplink hybridautomatic repeat request (HARQ). The ACK/NACK signal for uplink (UL)data on a PUSCH transmitted by the wireless device is transmitted on thePHICH.

A physical broadcast channel (PBCH) is transmitted in first four OFDMsymbols in a second slot of a first subframe of a radio frame. The PBCHcarries system information necessary for communication between thewireless device and a BS. The system information transmitted through thePBCH is referred to as a master information block (MIB). In comparisonthereto, system information transmitted on the PDCCH is referred to as asystem information block (SIB).

Control information transmitted through the PDCCH is referred to asdownlink control information (DCI). The DCI may include resourceallocation of the PDSCH (this is referred to as a downlink (DL) grant),resource allocation of a PUSCH (this is referred to as an uplink (UL)grant), a set of transmit power control commands for individual UEs inany UE group, and/or activation of a voice over Internet protocol(VoIP).

In 3GPP LTE/LTE-A, transmission of a DL transport block is performed ina pair of the PDCCH and the PDSCH. Transmission of a UL transport blockis performed in a pair of the PDCCH and the PUSCH. For example, thewireless device receives the DL transport block on a PDSCH indicated bythe PDCCH. The wireless device receives a DL resource assignment on thePDCCH by monitoring the PDCCH in a DL subframe. The wireless devicereceives the DL transport block on a PDSCH indicated by the DL resourceassignment.

FIG. 3 is a block diagram showing a structure of a PDCCH.

The 3GPP LTE/LTE-A uses blind decoding for PDCCH detection. The blinddecoding is a scheme in which a desired identifier is de-masked from acyclic redundancy check (CRC) of a received PDCCH (referred to as acandidate PDCCH) to determine whether the PDCCH is its own controlchannel by performing CRC error checking.

A BS determines a PDCCH format according to DCI to be transmitted to awireless device, attaches a cyclic redundancy check (CRC) to controlinformation, and masks a unique identifier (referred to as a radionetwork temporary identifier (RNTI)) to the CRC according to an owner orusage of the PDCCH (block 210).

If the PDCCH is for a specific wireless device, a unique identifier(e.g., cell-RNTI (C-RNTI)) of the wireless device may be masked to theCRC. Alternatively, if the PDCCH is for a paging message, a pagingindication identifier (e.g., paging-RNTI (P-RNTI)) may be masked to theCRC. If the PDCCH is for system information, a system informationidentifier (e.g., system information-RNTI (SI-RNTI)) may be masked tothe CRC. To indicate a random access response that is a response fortransmission of a random access preamble of the wireless device, arandom access-RNTI (RA-RNTI) may be masked to the CRC. To indicate atransmit power control (TPC) command for a plurality of wirelessdevices, a TPC-RNTI may be masked to the CRC.

When the C-RNTI is used, the PDCCH carries control information for aspecific wireless device (such information is called UE-specific controlinformation), and when other RNTIs are used, the PDCCH carries commoncontrol information received by all or a plurality of wireless devicesin a cell.

The CRC-attached DCI is encoded to generate coded data (block 220).Encoding includes channel encoding and rate matching.

The coded data is modulated to generate modulation symbols (block 230).

The modulation symbols are mapped to physical resource elements (REs)(block 240). The modulation symbols are respectively mapped to the REs.

A control region in a subframe includes a plurality of control channelelements (CCEs). The CCE is a logical allocation unit used to providethe PDCCH with a coding rate depending on a radio channel state, andcorresponds to a plurality of resource element groups (REGs). The REGincludes a plurality of REs. According to an association relation of thenumber of CCEs and the coding rate provided by the CCEs, a PDCCH formatand a possible number of bits of the PDCCH are determined.

One REG includes 4 REs. One CCE includes 9 REGs. The number of CCEs usedto configure one PDCCH may be selected from a set {1, 2, 4, 8}. Eachelement of the set {1, 2, 4, 8} is referred to as a CCE aggregationlevel.

The BS determines the number of CCEs used in transmission of the PDCCHaccording to a channel state. For example, a wireless device having agood DL channel state can use one CCE in PDCCH transmission. A wirelessdevice having a poor DL channel state can use 8 CCEs in PDCCHtransmission.

A control channel consisting of one or more CCEs performs interleavingon an REG basis, and is mapped to a physical resource after performingcyclic shift based on a cell identifier (ID).

FIG. 4 shows an example of monitoring a PDCCH. The section 9 of 3GPP TS36.213 V10.2.0 (2011-06) can be incorporated herein by reference.

The 3GPP LTE uses blind decoding for PDCCH detection. The blind decodingis a scheme in which a desired identifier is de-masked from a CRC of areceived PDCCH (referred to as a candidate PDCCH) to determine whetherthe PDCCH is its own control channel by performing CRC error checking. Awireless device cannot know about a specific position in a controlregion in which its PDCCH is transmitted and about a specific CCEaggregation or DCI format used for PDCCH transmission.

A plurality of PDCCHs can be transmitted in one subframe. The wirelessdevice monitors the plurality of PDCCHs in every subframe. Monitoring isan operation of attempting PDCCH decoding by the wireless deviceaccording to a format of the monitored PDCCH.

The 3GPP LTE uses a search space to reduce a load of blind decoding. Thesearch space can also be called a monitoring set of a CCE for the PDCCH.The wireless device monitors the PDCCH in the search space.

The search space is classified into a common search space and aUE-specific search space. The common search space is a space forsearching for a PDCCH having common control information and consists of16 CCEs indexed with 0 to 15. The common search space supports a PDCCHhaving a CCE aggregation level of {4, 8}. However, a PDCCH (e.g., DCIformats 0, 1A) for carrying UE-specific information can also betransmitted in the common search space. The UE-specific search spacesupports a PDCCH having a CCE aggregation level of {1, 2, 4, 8}.

Table 1 shows the number of PDCCH candidates monitored by the wirelessdevice.

TABLE 1 Number of Search Space Aggregation Size PDCCH DCI Type level L[In CCEs] candidates formats UE-specific 1 6 6 0, 1, 1A,1B, 2 12 6 1D,2, 2A 4 8 2 8 16 2 Common 4 16 4 0, 1A, 1C, 3/3A

A size of the search space is determined by Table 1 above, and a startpoint of the search space is defined differently in the common searchspace and the UE-specific search space. Although a start point of thecommon search space is fixed irrespective of a subframe, a start pointof the UE-specific search space may vary in every subframe according toa UE identifier (e.g., C-RNTI), a CCE aggregation level, and/or a slotnumber in a radio frame. If the start point of the UE-specific searchspace exists in the common search space, the UE-specific search spaceand the common search space may overlap with each other.

In a CCE aggregation level L∈{1,2,3,4}, a search space S^((L)) _(k) isdefined as a set of PDCCH candidates. A CCE corresponding to a PDCCHcandidate m of the search space S^((L)) _(k) is given by Equation 1below.L·{(Y_(k)+m′)mod └N_(CCE,k)/L┘}+i  [Equation 1]

Herein, i=0, 1, . . . , L−1, m=0, . . . , M^((L))−1, and N_(CCE,k)denotes the total number of CCEs that can be used for PDCCH transmissionin a control region of a subframe k. The control region includes a setof CCEs numbered from 0 to N_(CCE,k)−1. M^((L)) denotes the number ofPDCCH candidates in a CCE aggregation level L of a given search space.

If a carrier indicator field (CIF) is set to the wireless device,m′=m+M^((L))n_(cif). Herein, n_(cif) is a value of the CIF. If the CIFis not set to the wireless device, m′=m.

In a common search space, Y_(k) is set to 0 with respect to twoaggregation levels L=4 and L=8.

In a UE-specific search space of the aggregation level L, a variableY_(k) is defined by Equation 2 below.Y _(k)=(A·Y _(k−1))modD  [Equation 2]

Herein, Y⁻¹=n_(RNTI)≠0, A=39827, D=65537, k=floor(n_(s)/2), and n_(s)denotes a slot number in a radio frame.

When the wireless device monitors the PDCCH by using the C-RNTI, asearch space and a DCI format used in monitoring are determinedaccording to a transmission mode of the PDSCH. Table 2 below shows anexample of PDCCH monitoring in which the C-RNTI is set.

TABLE 2 Transmission Transmission mode of PDSCH based on mode DCI formatsearch space PDCCH Mode 1 DCI format 1A common and UE Single antennaport, port 0 specific DCI format 1 UE specific Single antenna port, port0 Mode 2 DCI format 1A common and UE Transmit diversity specific DCIformat 1 UE specific Transmit diversity Mode 3 DCI format 1A common andUE Transmit diversity specific DCI format 2A UE specific CDD(CyclicDelay Diversity) or Transmit diversity Mode 4 DCI format 1A common andUE Transmit diversity specific DCI format 2 UE specific Closed-loopspatial multiplexing Mode 5 DCI format 1A common and UE Transmitdiversity specific DCI format 1D UE specific MU-MIMO(Multi-user MultipleInput Multiple Output) Mode 6 DCI format 1A common and UE Transmitdiversity specific DCI format 1B UE specific Closed-loop spatialmultiplexing Mode 7 DCI format 1A common and UE If the number of PBCHtransmission ports is 1, specific single antenna port, port 0, otherwiseTransmit diversity DCI format 1 UE specific Single antenna port, port 5Mode 8 DCI format 1A common and UE If the number of PBCH transmissionports is 1, specific single antenna port, port 0, otherwise, Transmitdiversity DCI format 2B UE specific Dual layer transmission (port 7 or8), or single antenna port, port 7 or 8

The usage of the DCI format is classified as shown in Table 3 below.

TABLE 3 DCI format Contents DCI format 0 It is used for PUSCHscheduling. DCI format 1 It is used for scheduling of one PDSCHcodeword. DCI format 1A It is used for compact scheduling and randomaccess process of one PDSCH codeword. DCI format 1B It is used in simplescheduling of one PDSCH codeword having precoding information. DCIformat 1C It is used for very compact scheduling of one PDSCH codeword.DCI format 1D It is used for simple scheduling of one PDSCH codewordhaving precoding and power offset information. DCI format 2 It is usedfor PDSCH scheduling of UEs configured to a closed-loop spatialmultiplexing mode. DCI format 2A It is used for PDSCH scheduling of UEsconfigured to an open-loop spatial multiplexing mode. DCI format 3 It isused for transmission of a TPC command of a PUCCH and a PUSCH having a2-bit power adjustment. DCI format 3A It is used for transmission of aTPC command of a PUCCH and a PUSCH having a 1-bit power adjustment. DCOformat 4 It is used for PUSCH scheduling of one UL cell in multipleantenna transmission mode.

FIG. 5 shows a structure of a UL subframe.

Referring to FIG. 5, the UL subframe can be divided into a controlregion and a data region in a frequency domain. A physical uplinkcontrol channel (PUCCH) for transmitting UL control information isallocated to the control region. A physical uplink shared channel(PUSCH) for transmitting data (optionally, control information can betransmitted together) is allocated to the data region. According to aconfiguration, the UE may simultaneously transmit the PUCCH and thePUSCH, or may transmit any one of the PUCCH and the PUSCH.

The PUCCH for one UE is allocated in an RB pair in a subframe. RBsbelonging to the RB pair occupy different subcarriers in each of a1^(st) slot and a 2^(nd) slot. A frequency occupied by the RBs belongingto the RB pair allocated to the PUCCH changes at a slot boundary. Thisis called that the RB pair allocated to the PUCCH is frequency-hopped ina slot boundary. By transmitting UL control information over timethrough different subcarriers, a frequency diversity gain can beobtained.

A hybrid automatic repeat request (HARQ) acknowledgement(ACK)/non-acknowledgment (NACK) and channel status information (CSI)indicating a DL channel status (e.g., channel quality indicator (CQI), aprecoding matrix index (PMI), a precoding type indicator (PTI), a rankindication (RI)) can be transmitted on the PUCCH. Periodic CSI can betransmitted through the PUCCH.

The PUSCH is mapped to an uplink shared channel (UL-SCH) which is atransport channel. UL data transmitted through the PUSCH may be atransport block which is a data block for the UL-SCH transmitted duringa TTI. The transport block may include user data. Alternatively, the ULdata may be multiplexed data. The multiplexed data may be obtained bymultiplexing CSI and a transport block for the UL-SCH. Examples of theCSI multiplexed to the data may include a CQI, a PMI, an RI, etc.Alternatively, the UL data may consist of only CSI. Periodic oraperiodic CSI can be transmitted through the PUSCH.

Now, HARQ in 3GPP LTE will be described.

The 3GPP LTE uses synchronous HARQ in UL transmission, and usesasynchronous HARQ in DL transmission. In the synchronous HARQ,retransmission timing is fixed. In the asynchronous HARQ, theretransmission timing is not fixed. That is, in the synchronous HARQ,initial transmission and retransmission are performed with an HARQperiod.

FIG. 6 shows UL synchronous HARQ in 3GPP LTE.

A wireless device receives an initial UL grant on a PDCCH 310 from a BSin an n^(th) subframe.

The wireless device transmits a UL transport block on a PUSCH 320 byusing the initial UL grant in an (n+4)^(th) subframe.

The BS sends an ACK/NACK signal for the UL transport block on a PHICH331 in an (n+8)^(th) subframe. The ACK/NACK signal indicates a receptionacknowledgement for the UL transport block. The ACK signal indicates areception success, and the NACK signal indicates a reception failure.When the ACK/NACK signal is the NACK signal, the BS may send aretransmission UL grant on a PDCCH 332, or may not send an additional ULgrant. Alternatively, retransmission of previous data may be suspendedand a UL grant may be sent for transmission of new data. In case the ACKsignal, the BS may send the UL grant for new transmission through thePDCCH. In addition, the BS may send the UL grant for retransmission (orretransmission UL grant). Upon receiving the retransmission UL grant,the wireless device ignores the ACK/NACK signal and follows aninstruction of the retransmission UL grant. This is because the UL granthas higher reliability since the ACK/NACK signal does not have CRC andthe UL grant has CRC.

When the UL grant is not received and the NACK signal is received, thewireless device sends a retransmission block on a PUSCH 340 in an(n+12)^(th) subframe. For the transmission of the retransmission block,if the retransmission UL grant is received on the PDCCH 332, thewireless device uses the retransmission UL grant, and if theretransmission UL grant is not received, the wireless device uses theinitial UL grant.

The BS sends an ACK/NACK signal for the UL transport block on a PHICH351 in an (n+16)^(th) subframe. When the ACK/NACK signal is the NACKsignal, the BS may send a retransmission UL grant on a PDCCH 352, or maynot send an additional UL grant.

After initial transmission is performed in the (n+4)^(th) subframe,retransmission is performed in the (n+12)^(th) subframe, and thussynchronous HARQ is performed with an HARQ period corresponding to 8subframes.

Therefore, in frequency division duplex (FDD) of 3GPP LTE, 8 HARQprocesses can be performed, and the respective HARQ processes areindexed from 0 to 7.

FIG. 7 shows a structure of a PHICH in 3GPP LTE.

One PHICH carries only 1-bit ACK/NACK corresponding to a PUSCH for oneUE, that is, corresponding to a single stream.

In step S310, the 1-bit ACK/NACK is coded into 3 bits by using arepetition code having a code rate of 1/3.

In step S320, the coded ACK/NACK is modulated using binary phase shiftkeying (BPSK) to generate 3 modulation symbols.

In step S330, the modulation symbols are spread by using an orthogonalsequence. A spreading factor (SF) is N^(PHICH) _(SF)=4 in a normal CP,and is N^(PHICH) _(SF)=2 in an extended CP. The number of orthogonalsequences used in the spreading is N^(PHICH) _(SF)*2 to apply I/Qmultiplexing. PHICHs which are spread by using N^(PHICH) _(SF)*2orthogonal sequences can be defined as one PHICH group.

Table 4 below shows an orthogonal sequence for the PHICH.

TABLE 4 orthogonal sequence normal CP extended CP n^(seq) _(PHICH)(N^(PHICH) _(SF) = 4) (N^(PHICH) _(SF) = 2) 0 [+1 +1 +1 +1] [+1 +1] 1[+1 −1 +1 −1] [+1 −1] 2 [+1 +1 −1 −1] [+j +j] 3 [+1 −1 −1 +1] [+j −j] 4[+j +j +j +j] 5 [+j −j +j −j] 6 [+j +j −j −j] 7 [+j −j −j +j]

In step S340, layer mapping is performed on the spread symbols.

In step S350, the layer-mapped symbols are transmitted by being mappedto resources.

A plurality of PHICHs mapped to resource elements of the same setconstitute a PHICH group. Each PHICH included in the PHICH group isidentified by a different orthogonal sequence. In the FDD system,N^(group) _(PHICH), i.e., the number of PHICH groups, is constant in allsubframes, and can be determined by Equation 3 below.

$\begin{matrix}{N_{PHICH}^{group} = \left\{ \begin{matrix}{{ceil}\left( {N_{g}\left( {N_{RB}^{DL}/8} \right)} \right)} & {{for}\mspace{14mu}{normal}\mspace{14mu}{CP}} \\{2\mspace{14mu}{{ceil}\left( {N_{g}\left( {N_{RB}^{DL}/8} \right)} \right)}} & {{for}\mspace{14mu}{extended}\mspace{14mu}{CP}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Herein, Ng denotes a parameter transmitted through a physical broadcastchannel (PBCH), where Ng∈{1/6, 1/2 ,1,2}. N^(DL) _(RB) denotes thenumber of DL RBs.

ceil(x) is a function for outputting a minimum value among integersequal to or greater than x. floor(x) is a function for outputting amaximum value among integers equal to or less than x.

The wireless device identifies a PHICH resource by using an index pair(n^(group) _(PHICH), n^(seq) _(PHICH)) used by the PHICH. A PHICH groupindex n^(group) _(PHICH) has a value in the range of 0 to N^(group)_(PHICH)−1. An orthogonal sequence index n^(seq) _(PHICH) denotes anindex of an orthogonal sequence.

An index pair (n^(group) _(PHICH), n^(seq) _(PHICH)) is obtainedaccording to Equation 1 below.n _(PHICH) ^(group)=(I _(PRB) _(_) _(RA) ^(lowest) ^(_) ^(index) +n_(DMRS))modN _(PHICH) ^(group) +I _(PHICH) N _(PHICH) ^(group)n _(PHICH) ^(seq)=(floor(I _(PRB) _(_) _(RA) ^(lowest) ^(_) ^(index) /N_(PHICH) ^(group))+n _(DRMS))mod 2N _(SF) ^(PHICH)  [Equation 4]

Herein, n_(DMRS) denotes a cyclic shift of a demodulation referencesignal (DMRS) within the most recent UL grant for a transport blockrelated to corresponding PUSCH transmission. The DMRS is an RS used forPUSCH transmission. N^(PHICH) _(SF) denotes an SF size of an orthogonalsequence used in PHICH modulation. I^(lowest) ^(_) ^(index) _(PRB) _(_)_(RA) denotes the smallest PRB index in a 1^(st) slot of correspondingPUSCH transmission. I_(PHICH) is 0 or 1.

A physical resource block (PRB) is a unit frequency-time resource fortransmitting data. One PRB consists of a plurality of contiguous REs ina frequency-time domain. Hereinafter, the RB and the PRB are used forthe same concept.

<Semi-Persistent Scheduling: SPS>

In the wireless communication system, the UE receives schedulinginformation such as a DL grant, a UL grant, etc., through the PDCCH, andperforms an operation of receiving the PDSCH and transmitting the PUSCHon the basis of the scheduling information. In general, the DL grant andthe PDSCH are received in the same subframe. In addition, in case ofFDD, the PUSCH is transmitted four subframes later than a subframe inwhich the UL grant is received. In addition to such dynamic scheduling,LTE also provides semi-persistent scheduling (SPS).

In a DL or UL SPS, a higher-layer signal such as radio resource control(RRC) can be used to report a UE about specific subframes in whichsemi-persistent transmission/reception is performed. Examples of aparameter given by using the higher layer signal may be a subframeperiod and an offset value.

The UE recognizes semi-persistent transmission through RRC signaling,and thereafter performs or releases SPS PDSCH reception or SPS PUCCHtransmission upon receiving an activation or release signal of SPStransmission through a PDCCH. That is, in a case where the activation orrelease signal is received through the PDCCH instead of directlyperforming SPS transmission even if SPS scheduling is assigned throughRRC signaling, SRS transmission and reception are performed in asubframe corresponding to an offset and a subframe period allocatedthrough RRC signaling by applying a modulation and coding rate based onmodulation and coding scheme (MCS) information and a frequency resource(or resource block) based on resource block allocation designated in thePDCCH. If an SPS release signal is received through the PDCCH, SPStransmission/reception is suspended. Upon receiving a PDCCH includingthe SPS activation signal, the suspended SPS transmission/reception isresumed by using an MCS and a frequency resource designated in thePDCCH.

<Carrier Aggregation>

Now, a carrier aggregation system will be described.

FIG. 8 shows an example of comparing a legacy single-carrier system anda carrier aggregation system.

Referring to FIG. 8, only one carrier is supported for a UE in an uplinkand a downlink in the single-carrier system. Although the carrier mayhave various bandwidths, only one carrier is assigned to the UE.Meanwhile, multiple component carriers (CCs) (i.e., DL CCs A to C and ULCCs A to C) can be assigned to the UE in the carrier aggregation (CA)system. A CC implies a carrier used in a carrier aggregation system, andcan be simply referred to as a carrier. For example, three 20 MHz CCscan be assigned to allocate a 60 MHz bandwidth to the UE.

The carrier aggregation system can be divided into a contiguous carrieraggregation system in which carriers are contiguous to each other and anon-contiguous carrier aggregation system in which carriers areseparated from each other. Hereinafter, when it is simply called thecarrier aggregation system, it should be interpreted such that bothcases of contiguous CCs and non-contiguous CCs are included.

A CC which is a target when aggregating one or more CCs can directly usea bandwidth that is used in the legacy system in order to providebackward compatibility with the legacy system. For example, a 3GPP LTEsystem can support a carrier having a bandwidth of 1.4 MHz, 3 MHz, 5MHz, 10 MHz, 15 MHz, and 20 MHz, and a 3GPP LTE-A system can configure abroadband of 20 MHz or higher by using each carrier of the 3GPP LTEsystem as a CC. Alternatively, the broadband can be configured bydefining a new bandwidth without having to directly use the bandwidth ofthe legacy system.

A frequency band of a wireless communication system is divided into aplurality of carrier frequencies. Herein, the carrier frequency impliesa center frequency of a cell. Hereinafter, the cell may imply a downlinkfrequency resource and an uplink frequency resource. Alternatively, thecell may also imply combination of a downlink frequency resource and anoptional uplink frequency resource. In general, if carrier aggregation(CA) is not considered, uplink and downlink frequency resources canalways exist in pair in one cell.

In order to transmit and receive packet data through a specific cell,the UE first has to complete a configuration of the specific cell.Herein, the configuration implies a state of completely receiving systeminformation required for data transmission and reception for the cell.For example, the configuration may include an overall procedure thatrequires common physical layer parameters necessary for datatransmission and reception, media access control (MAC) layer parameters,or parameters necessary for a specific operation in a radio resourcecontrol (RRC) layer. A cell of which configuration is complete is in astate capable of immediately transmitting and receiving a packet uponreceiving only information indicating that packet data can betransmitted.

The cell in a state of completing its configuration can exist in anactivation or deactivation state. Herein, the activation implies thatdata transmission or reception is performed or is in a ready state. TheUE can monitor or receive a control channel (i.e., PDCCH) and a datachannel (i.e., PDSCH) of an activated cell in order to confirm aresource (e.g., frequency, time, etc.) allocated to the UE.

The deactivation implies that transmission or reception of traffic datais impossible and measurement or transmission/reception of minimuminformation is possible. The UE can receive system information (SI)required for packet reception from a deactivated cell. On the otherhand, the UE does not monitor or receive a control channel (i.e., PDCCH)and a data channel (i.e., PDSCH) of the deactivated cell in order toconfirm a resource (e.g., frequency, time, etc.) allocated to the UE.

A cell can be classified into a primary cell, a secondary cell, aserving cell, etc.

The primary cell implies a cell which operates at a primary frequency,and also implies a cell which performs an initial connectionestablishment procedure or a connection re-establishment procedure or acell indicated as the primary cell in a handover procedure.

The secondary cell implies a cell which operates at a secondaryfrequency, and is configured when an RRC connection is once establishedand is used to provide an additional radio resource.

The serving cell is configured with the primary cell in case of a UE ofwhich carrier aggregation is not configured or which cannot provide thecarrier aggregation. If the carrier aggregation is configured, the term‘serving cell’ is used to indicate a cell configured for the UE, and thecell may be plural in number. One serving cell may consist of one DL CCor a pair of {DL CC, UL CC}. A plurality of serving cells may beconfigured with a set consisting of a primary cell and one or aplurality of cells among all secondary cells.

A primary component carrier (PCC) denotes a CC corresponding to theprimary cell. The PCC is a CC that establishes an initial connection (orRRC connection) with a BS among several CCs. The PCC serves forconnection (or RRC connection) for signaling related to a plurality ofCCs, and is a CC that manages a UE context which is connectioninformation related to the UE. In addition, the PCC establishes aconnection with the UE, and thus always exists in an activation statewhen in an RRC connected mode. A downlink CC corresponding to theprimary cell is called a downlink primary component carrier (DL PCC),and an uplink CC corresponding to the primary cell is called an uplinkprimary component carrier (UL PCC).

A secondary component carrier (SCC) denotes a CC corresponding to asecondary cell. That is, the SCC is a CC allocated to the UE in additionto the PCC. The SCC is an extended carrier used by the UE for additionalresource allocation or the like in addition to the PCC, and can be in anactivation state or a deactivation state. A DL CC corresponding to thesecondary cell is called a DL secondary CC (SCC). A UL CC correspondingto the secondary cell is called a UL SCC.

The primary cell and the secondary cell have the following features.

First, the primary cell is used for PUCCH transmission. Second, theprimary cell is always activated, whereas the secondary cell isactivated/deactivated according to a specific condition. Third, when theprimary cell experiences a radio link failure (RLF), RRCre-establishment is triggered. Fourth, the primary cell can be changedby a handover procedure accompanied by a random access channel (RACH)procedure or security key modification. Fifth, non-access stratum (NAS)information is received through the primary cell. Sixth, in case of anFDD system, the primary cell always consists of a pair of a DL PCC and aUL PCC. Seventh, for each UE, a different CC can be configured as theprimary cell. Eighth, the primary cell can be replaced only through ahandover, cell selection/cell reselection procedure. When adding a newsecondary cell, RRC signaling can be used for transmission of systeminformation of a dedicated secondary cell.

Regarding a CC constituting a serving cell, a DL CC can construct oneserving cell. Further, the DL CC can be connected to a UL CC toconstruct one serving cell. However, the serving cell is not constructedonly with one UL CC.

Activation/deactivation of a CC is equivalent to the concept ofactivation/deactivation of a serving cell. For example, if it is assumedthat a serving cell 1 consists of a DL CC 1, activation of the servingcell 1 implies activation of the DL CC 1. If it is assumed that aserving cell 2 is configured by connecting a DL CC 2 and a UL CC 2,activation of the serving cell 2 implies activation of the DL CC 2 andthe UL CC 2. In this sense, each CC can correspond to a cell.

The number of CCs aggregated between a downlink and an uplink may bedetermined differently. Symmetric aggregation is when the number of DLCCs is equal to the number of UL CCs. Asymmetric aggregation is when thenumber of DL CCs is different from the number of UL CCs. In addition,the CCs may have different sizes (i.e., bandwidths). For example, if 5CCs are used to configure a 70 MHz band, it can be configured such as 5MHz CC(carrier #0 )+20 MHz CC(carrier #1)+20 MHz CC(carrier #2)+20 MHzCC(carrier #3)+5 MHz CC(carrier #4).

As described above, the carrier aggregation system can support multiplecomponent carriers (CCs), that is, multiple serving cells, unlike asingle-carrier system.

The carrier aggregation system can support cross-carrier scheduling. Thecross-carrier scheduling is a scheduling method capable of performingresource allocation of a PDSCH transmitted by using a different carrierthrough a PDCCH transmitted via a specific CC and/or resource allocationof a PUSCH transmitted via another CC other than a CC basically linkedto the specific CC. That is, the PDCCH and the PDSCH can be transmittedthrough different DL CCs, and the PUSCH can be transmitted via a UL CCother than a UL CC linked to a DL CC on which a PDCCH including a ULgrant is transmitted. As such, in a system supporting the cross-carrierscheduling, a carrier indicator is required to report a specific DLCC/UL CC used to transmit the PDSCH/PUSCH for which the PDCCH providescontrol information. A field including the carrier indicator ishereinafter called a carrier indication field (CIF).

The carrier aggregation system supporting the cross-carrier schedulingmay include a CIF in the conventional downlink control information (DCI)format. In a system supporting the cross-carrier scheduling, e.g., anLTE-A system, the CIF is added to the conventional DCI format (i.e., theDCI format used in LTE) and thus the number of bits can be extended by 3bits, and the PDCCH structure can reuse the conventional coding scheme,resource allocation scheme (i.e., CCE-based resource mapping), etc.

FIG. 9 shows an example of cross-carrier scheduling in a carrieraggregation system.

Referring to FIG. 9, a BS can configure a PDCCH monitoring DL CC set.The PDCCH monitoring DL CC set consists of some DL CCs among allaggregated DL CCs. When the cross-carrier scheduling is configured, a UEperforms PDCCH monitoring/decoding only for a DL CC included in thePDCCH monitoring DL CC set. In other words, the BS transmits a PDCCH fora to-be-scheduled PDSCH/PUSCH only via a DL CC included in the PDCCLmonitoring DL CC set. The PDCCH monitoring DL CC set can be determinedin a UE-specific, UE group-specific, or cell-specific manner.

In the example of FIG. 9, 3 DL CCs (i.e., DL CC A, DL CC B, DL CC C) areaggregated, and the DL CC A is determined as the PDCCH monitoring DL CC.The UE can receive a DL grant for a PDSCH of the DL CC A, the DL CC B,and the DL CC C through the PDCCH. A CIF may be included in DCItransmitted through the PDCCH of the DL CC A to indicate a specific DLCC for which the DCI is provided.

FIG. 10 shows an example of scheduling when cross-carrier scheduling isconfigured in a carrier aggregation system.

Referring to FIG. 10, a DL CC 0, a DL CC 2, and a DL CC 4 constitute amonitoring DL CC set. A UE searches for a DL grant/UL grant regardingthe DL CC 0 and a UL CC 0 (i.e., a UL CC linked to the DL CC 0 by usingan SIB2) in a CSS of the DL CC 0. Further, the UE searches for a DLgrant/UL grant regarding a DL CC 1 and a UL CC 1 in an SS 1 of the DL CC0. The SS 1 is an example of a USS. That is, the SS 1 of the DL CC 0 isa search space for searching for the DL grant/UL grant for performingcross-carrier scheduling.

Now, the present invention will be described.

In a system enhanced from LTE release 10, a greater number of UEs canaccess to one BS in comparison with the legacy system due to a techniquesuch as machine type communication (MTC), enhanced multi user-multiinput multi output (MU-MIMO), etc. In this case, it may be difficult todeliver control information to a plurality of UEs by using only theexisting control region, i.e., a PDCCH region, in a DL subframe. Thatis, the control region may be insufficient. In addition, a plurality ofRRHs or the like are deployed in a cell, which may cause a problem of aninterference in the control region.

The LTE-A system considers to introduce a new control channel to solve aresource shortage problem of a PDCCH which is a channel for transmittingcontrol information and a reception performance deterioration problem ofa PDCCH region caused by an interference. For convenience ofexplanation, the new control channel is called an enhanced-PDCCH(E-PDCCH).

The conventional PDCCH differs from the E-PDCCH as follows.

1) The conventional PDCCH may be located in a control region in asubframe, that is, a region consisting of first N OFDM symbols (where Nis any natural number in the range of 1 to 4), whereas the E-PDCCH maybe located in a data region in the subframe, that is, a regionconsisting of the remaining OFDM symbols other than the N OFDM symbols.

2) The conventional PDCCH can be decoded on the basis of a cell-specificreference signal, i.e., CRS, that can be received by all UEs in a cell,whereas the E-PDCCH can be decoded on the basis of not only the CRS butalso a DM-RS which is specific to a particular UE. Therefore, similarlyto the PDSCH, beamforming can be applied to the E-PDSCH by usingprecoding, and as a result, a reception SINR may be increased.

3) The conventional PDCCH may be applied to a UE which operates in LTE,whereas the E-PDCCH may be selectively applied to a UE supporting LTE-A.Of course, the UE supporting the LTE-A may also support the conventionalPDCCH.

In terms of resources constituting the E-PDDCH, there may be adistributed E-PDCCH consisting of distributed resources and a localizedE-PDCCH consisting of localized resources. The distributed E-PDCCH canacquire a diversity gain and can be used to transmit control informationfor several UEs. The distributed E-PDCCH has a frequency selectiveproperty and can be used to transmit control information for aparticular UE.

Meanwhile, in the LTE-A, a greater amount of ACK/NACK may be transmittedand an interference may become severe in comparison with the legacysystem such as a multi-node system in which multiple nodes are includedin a cell, a carrier aggregation system supporting multiple carriers,etc. Therefore, a PHICH may also have a resource shortage problem and areception performance deterioration problem caused by an interference.To solve these problems, the LTE-A considers to introduce a new PHICH inaddition to the conventional PHICH. For convenience of explanation, thenew PHICH is called an enhanced-PHICH (E-PHICH). The PHICH and theE-PHICH are channels on which a BS transmits ACK/NACK for a UL datachannel transmitted by a UE. Unlike a case where the PHICH is configuredin the PDCCH region, the E-PHICH may be configured in the PDSCH region.For example, the E-PHCIH may be configured in the E-PDCCH regionconfigured in the PDSCH region.

FIG. 11 shows an example of configuring an E-PHICH region and an E-PDCCHregion.

Referring to FIG. 11, the E-PDCCH region may be configured in a PDSCHregion.

Similarly to the PDCCH region, the E-PDCCH region may include anenhanced-common search space (E-CSS) in which all UEs or a specific UEgroup in a cell search for an E-PDCCH thereof and anenhanced-UE-specific search space (E-USS) in which only a specific UEsearches for an E-PDCCH thereof. Alternatively, any one of the E-CSS andthe E-USS may be included.

Meanwhile, the E-PHICH may be configured in the E-PDCCH region. Forexample, the E-PHICH may be configured in the E-CSS. In this case, theE-PHICH may be used to transmit ACK/NACK for a plurality of UEs throughmultiplexing.

<Configuration of Start OFDM Symbol of E-PHICH>

Similarly to the E-PDCCH, if the E-PHICH is configured in the PDSCHregion and is configured from a first slot of a subframe, a position ofstarting an OFDM symbol of the E-PHICH must be determined by consideringthe number of all OFDM symbols in which the PDCCH can be located.

For example, the start OFDM symbol position of the E-PHICH may be setequal to a start position of: 1) an E-PDCCH of the same cell or the samesubframe; or 2) an E-PDCCH of a subframe in which a UL grant istransmitted for scheduling a PUSCH which is a target of ACK/NACKtransmitted on the E-PHICH. This is because the E-PDCCH and the E-PHICHhave a similar property in that they are transmitted in a region otherthan the conventional PDCCH region, and this is to decrease anadditional signaling or procedure for the start position of the E-PHICH.

If an E-PDCCH including a CSS and an E-PDCCH including a USS are eachpresent in a specific cell and if start positions of the two can be setdifferently, the start position of the E-PHICH may be set equal to thestart position of the E-PDCCH including the CSS.

In the presence of the distributed E-PDCCH and the localized E-PDCCH,the start position of the E-PHICH may be set identical to thedistributed E-PDCCH.

If the configuration of the E-PHICH region is applied commonly to aplurality of UEs, the start position of the E-PHICH may be an OFDMsymbol next to a maximum OFDM symbol in which the PDCCH can be located.For example, if the PDCCH can be located in up to 3 OFDM symbols as to aspecific band, the start position of the E-PHICH may be a 4^(th) OFDMsymbol.

Alternatively, the start position of the E-PHICH may be configured byusing an RRC message.

In several methods described above, the E-PHICH is characterized interms of a common channel since ACK/NACK information for a plurality ofUEs is multiplexed. In this case, it is considered an aspect in which atransmission method capable of obtaining a diversity gain that can beobtained for an unspecific UE is advantageous over a case of obtaining afrequency selective gain for a specific UE. A more conservative andreliable method for this is to determine a start position of the E-PHICHby considering a maximum range of the PDCCH. The RRC message isconfigured to provide flexibility of resource utilization to the BS.

Alternatively, the E-PHICH may be transmitted on a search space for anE-PDCCH for scheduling a PUSCH or may be allocated to a specificposition (e.g., a pre-fixed position or a position signaled by the RRCmessage) of the search space.

For example, when a search space of a DL grant and a search space of aUL grant are configured independently, the E-PHICH may be located in thesearch space of the UL grant. An ACK/NACK response of the BS for thePUSCH may cause retransmission of a PUSCH without a UL grant from aperspective of the UE, and thus may be used instead of the UL grant.Therefore, the E-PHICH may be preferably located in the search space ofthe UL grant. In addition, this method is also advantageous in terms ofload balancing of the E-PDCCH for the UL grant and the DL grant.

FIG. 12 shows an example of allocating an E-PHICH in a search space.

Referring to FIG. 12, a search space of a DL grant and a search space ofa UL grant may be subjected to time division multiplexing (TDM). Forexample, the search space of the DL grant may be located in a firstslot, and the search space of the UL grant may be located in a secondslot. In this case, the E-PHICH is also preferably located in the secondslot.

The search space of the DL grant and the search space of the UL grantare subjected to the TDM in that order because an HARQ process based onthe UL grant is more tolerant of a time delay since PDSCH decoding basedon the DL grant occurs in the same subframe whereas the HARQ processbased on the UL grant occurs after a specific number of subframes.Therefore, the E-PHICH is also preferably allowed to maintain the sameHARQ timing as the UL grant.

Meanwhile, similarly to a PDCCH-PHICH relation, the E-PHICH may beconfigured with a channel independent of the E-PDCCH.

Alternatively, instead of configuring the E-PHICH as an independentchannel, transmission may be performed in a DCI format of the E-PDCCH.That is, instead of transmitting ACK/NACK for a PUSCH through anadditional control channel, i.e., E-PHICH, it may be transmitted bybeing included in the DCI format of the e-PDCCH or may be transmitted bydefining a new DCI format. In this case, the DCI format (or new DCIformat) may include multiplexing information of ACK/NACK for a pluralityof UEs, and the BS may transmit it by scrambling CRC of the DCI formatby the use of an E-PHICH identifier (also called as an E-PHICH RNTI)allocated to a specific UE group. If ACK/NACK for the plurality of UEsis multiplexed to configure a bit-stream, each UE may receive ACK/NACKinformation through a bit field of a pre-signaled position.Alternatively, as to only a specific UE for transmitting a PUSCH whichis a target of ACK/NACK, it may be transmitted by scrambling CRC with aC-RNTI allocated to the specific UE according to a compact DCI formatincluding the ACK/NACK without scheduling information (TPC may beincluded) such as resource allocation information (including frequencyhopping) or a new data indicator (NDI), a modulation and coding scheme(MCS), a DMRS, etc.

<Selection of PHICH or E-PHICH and Selection of Cell for ACK/NACK forPUSCH>

Even if both of the PHICH and the E-PHICH are supported in a wirelesscommunication system, only one of the PHICH and the E-PHICH may beconfigured for each cell or for each subframe, or both of them may beconfigured.

If the PHICH and the E-PHICH can be configured in the subframe, the UEmay monitor both of the PHICH and the E-PHICH to receive ACK/NACK for aPUSCH, which may be ineffective and may increase power consumption ofthe UE.

Hereinafter, for convenience of explanation, from a perspective of theUE, a cell for monitoring the PDCCH is called a PDCCH cell, a cell formonitoring the E-PDCCH is called an E-PDCCH cell, a cell fortransmitting the PHICH is called a PHICH cell, and a cell fortransmitting the E-PHICH is called an E-PHICH cell.

The PDCCH cell may be a cell in which a search space is configured inthe PDCCH region, and the E-PDCCH cell may be a cell in which the searchspace is configured in the E-PDCCH region. The PDCCH cell and theE-PDCCH cell may be mutually exclusive or may overlap with each other.The PHICH cell and the E-PHICH cell also may be mutually exclusive ormay overlap with each other. That is, in one cell, the UE may beconfigured to monitor the PHICH in some subframes, and the UE may beconfigured to monitor the E-PHICH in other subframes. That is, anoperation described below may differ for each subframe.

Now, a case where monitoring of an E-PHICH is not configured and a casewhere monitoring of the E-PHICH is configured are described belowdistinctively.

I. When it is configured that a UE does not monitor an E-PHICH.

1. First embodiment: In a case where a UL grant exists in a PDCCH.

1) A corresponding PDCCH cell is a PHICH cell. That is, a PHICH istransmitted together in a cell in which the PDCCH is transmitted.Alternatively, 2) the PHICH cell may be designated with RRC. That is, aBS may configure a cell in which the PHICH is transmitted to a UEthrough an RRC message. In this case, the PHICH cell and the PDCCH cellmay be configured independently. This may be preferable for aconsistence with a method of designating a PHICH transmission cell byusing RRC as to an E-PDCCH cell to be described below.

2. Second embodiment: In a case where a UL grant exists in an E-PDCCH.

1) Embodiment 2-1: When a plurality of cells are configured to a UE, aPDCCH cell among the plurality of cells may be a PHICH cell. If thePDCCH cell is plural in number, the PHICH cell may be a primary cell.

Since a cell having a relatively good channel state is selected as thePDCCH cell, the PHICH cell is selected from PDCCH cells so that the UEcan reliably receive the PHICH. In particular, since the primary cellperforms decoding of a PDCCH region in system information reception andinitial access, a cell which is examined for PDCCH reception isselected.

2) Embodiment 2-2: A PHICH cell may be a cell in which a UL grant istransmitted through an E-PDCCH. That is, a BS may transmit a PHICHthrough a cell in which the UL grant is transmitted. This will bedescribed with reference to FIG. 13. The method of FIG. 13 transmitsACK/NACK through the PHICH if an interference of a PDCCH region in whicha PHICH is transmitted is not significant in a cell in which the E-PDCCHis transmitted. PHICHs in neighboring cells may be shifted in afrequency axis on the basis of a cell ID, and due to such acharacteristic, the a PHICH in a specific cell in which a UL grant istransmitted on the E-PDCCH utilizes a PHICH resource if an interferencefrom the neighboring cell is not significant. According to this method,an operation irrespective of a reconfiguration of RRC signaling may beperformed.

FIG. 13 shows a method of receiving ACK/NACK according to the embodiment2-2.

Referring to FIG. 13, DL CCs 0 and 1 and UL CCs 0 and 1 may beconfigured to a UE. The DL CC 0 and the UL CC 0 configure a firstserving cell, and the DL CC 1 and the UL CC 1 configure a second servingcell. The DL CCs 0 and 1 and the UL CCs 0 and 1 are only for indexing ofrespective CCs for convenience of explanation (the same is also appliedto the figures below). The UE may receive UL grants for the UL CCs 0 and1 through an E-PDCCH of the DL CC 0. The UE transmits a PUSCH throughthe UL CCs 0 and 1 according to the UL grants. ACK/NACK for the PUSCH isreceived through a PHICH of the DL CC 0. In FIG. 13, the UL grants andthe PHICH are indicated in the same subframe for convenience ofexplanation only, and thus they are not necessarily transmittedsimultaneously in the same subframe.

3) Embodiment 2-3: A PHICH cell for a PUSCH may be pre-designated withRRC. This will be described with reference to FIG. 14.

FIG. 14 shows an example of a method of configuring a PHICH cell andreceiving ACK/NACK.

Referring to FIG. 14, a BS transmits to a UE an RRC message includingPHICH cell indicator information indicating a PHICH cell (step S110).The RRC message may be an ‘RRCConnectionReconfiguration message’. It isassumed that the PHICH cell indicated by the PHICH cell indicatorinformation is a first cell.

The BS transmits a UL grant through a second cell (step S120). The ULgrant may be transmitted through a PDCCH, and may be transmitted throughan E-PDCCH.

The UE transmits a PUSCH on the basis of the UL grant (step S130).

The BS transmits ACK/NACK for the PUSCH through the first cell (stepS140). Since the PHICH cell (i.e., first cell) can be known by using thePHICH cell indicator information included in the RRC message, the UE canreceive ACK/NACK for the PUSCH through the first cell.

That is, the method described with reference to FIG. 14 differs from themethod of FIG. 13 in that the PHICH cell is explicitly indicated. Inaddition, there is no restriction in that the PHICH cell must beconfigured identically to the E-PDCCH cell in which the UL grant istransmitted.

A configuration based on the RRC may be indicated for each cell. Inaddition, it may be configured differently for each subframe in one cellso that an inter-cell interference situation for each subframe is moreeffectively applied than other situations.

4) Embodiment 2-4: Unlike methods of transmitting ACK/NACK through thePHICH, the PHICH for the PUSCH may not be transmitted. In this case,retransmission of HARQ may be performed only by a UL grant.Conventionally, if a UL grant does not exist when NACK is receivedthrough the PHICH, the UE retransmits a PUSCH by using a resource basedon a previous UL grant. However, the present invention may not allowHARQ retransmission based on NACK and may allow HARQ retransmissionbased on only the UL grant.

The UE may determine whether to transmit a new PUSCH or retransmit aPUSCH on the basis of a new data indicator (NDI) included in the ULgrant. That is, if the NDI of the UL grant indicates new PUSCHtransmission, it may be assumed that the UE receives ACK for apreviously transmitted PUSCH. A BS may use an RRC message topredetermine whether to operate without a PHICH or to transmit thePHICH.

The PDCCH and the PHICH may not exist in a new carrier type (NCT). Inthis case, the aforementioned embodiments 2-1, 2-3, and 2-4 may beapplied. In addition, the embodiment 2-2 may be applied in the existinglegacy carrier type (LCT). The aforementioned embodiments 2-1, 2-3, and2-4 may be more appropriate to an NCT in which a PHICH cannot beconfigured since CRS is not configured. For example, when the E-PDCCH istransmitted in the NCT, since the PHICH cannot be configured in the NCT,the method of the embodiment 2-2 cannot be used, and the method of theembodiments 2-1, 2-3, and 2-4 is required.

In case of the embodiments 2-2 and 2-3, if a corresponding cell is not aprimary cell, the PHICH may be configured by using a cell ID signaled byRRC, the number of reference signal antenna ports, Ng, and a PHICHduration.

The embodiment 2-2 may be configured when an E-PDCCH cell in which a ULgrant is transmitted is a PDCCH cell. That is, the cell corresponds to acell which monitors a PDCCH in some subframes and monitors an E-PDCCH inother subframes. This is because there is a case where such a cell usesthe E-PDCCH for the purpose of compensating for a capacity shortage ofthe PDCCH since an interference of the PDCCH is not significant.

In addition, a method may be used in which a PHICH is used in case of aUL HARQ process scheduled with a PDCCH and in which the PHICH is notpresent in case of a UL HARQ process scheduled with an E-PDCCH, that is,the embodiment 2-4 may be used. Alternatively, the embodiment 2-2 may beused when a subframe in which a retransmission UL grant will be receivedis a subframe in which monitoring of a USS of a PDCCH is configured, andthe embodiment 2-1, 2-3, or 2-4 may be applied when it is a subframe inwhich monitoring of a USS of an E-PDCCH is configured and in which thesubframe does not have the E-PHICH. Alternatively, the embodiment 2-2may be used when a subframe in which an ACK/NACK response for a PUSCHwill be received is a subframe in which monitoring of a USS of a PDCCHis configured, and the embodiment 2-1, 2-3, or 2-4 may be applied whenit is a subframe in which monitoring of a USS of an E-PDCCH isconfigured and in which the subframe does not have the E-PHICH. It isconsidered herein a case where PUSCH subframe bundling is applied or acase where UL grant timing differs from PHICH timing occurs whendifferent TDD UL-DL configurations are used.

II. Third embodiment: When it is configured that a UE monitors anE-PHICH.

When an E-PHICH is configured through a higher layer signal, a PHICH andthe E-PHICH may co-exist in the same subframe. Therefore, a BS mayreport a UE about which channel is used between the PHICH and theE-PHICH to transmit ACK/NACK. The PHICH and the E-PHICH are selectivelyused according to respective properties. The PHICH is located in a PDCCHregion, and thus it may be difficult to avoid performance deteriorationwhen an interference caused by a PDCCH region of a neighboring cell issignificant. When the E-PHICH is configured, there is a disadvantage inthat an additional PDSCH resource is consumed. However, since theE-PHICH can be configured in a PDSCH region, there is an advantage inthat an inter-cell interference can be avoided by regulating inter-cellPDSCH scheduling.

1) Embodiment 3-1: The BS may report about which channel is used betweenthe PHICH and the E-PHICH to transmit ACK/NACK for the PUSCH by using anRRC message for each subframe. A PHICH monitoring configuration and anE-PHICH monitoring configuration may be performed in the same subframeas a PDCCH monitoring configuration and an E-PDCCH monitoringconfiguration, respectively.

2) Embodiment 3-2: Alternatively, the selection of the PHICH and theE-PHICH may be determined according to a DCI format used for a UL grant.For example, the PHICH may be used for a PUSCH scheduled with a DCIformat 0, and the E-PHICH may be used for a PUSCH scheduled with a DCIformat 4. The UE may implicitly know which channel is used between thePHICH and the E-PHICH to receive ACK/NACK on the basis of the DCI formatincluded in the UL grant.

3) Embodiment 3-3: The selection of the PHICH and the E-PHICH may beindicated by using a bit field combination of the UL grant. For example,a specific state of a DMRS field may be allowed to instruct the use ofthe E-PHICH.

4) Embodiment 3-4: When a UL grant for a corresponding HARQ process istransmitted, the E-PHICH may not be transmitted. Therefore, when the ULgrant is detected, even if there is a resource allocated with theE-PHICH, the UE may ignore the resource and may utilize it as a PDSCH.

Fourth Embodiment: Designation of a PHICH cell/subframe or an E-PHICHcell/subframe.

Embodiment 4-1: If a UL grant exists in a PDCCH, a PDCCH cell (orsubframe) may be a PHICH cell (or subframe), and if the UL grant existsin an E-PDCCH, an E-PDCCH cell (or subframe) may be an E-PHICH cell (orsubframe). This will be described with reference to FIG. 15.

FIG. 15 shows an example of a method of receiving ACK/NACK of a UE.

Referring to FIG. 15, DL CCs 0 and 1 and UL CCs 0 and 1 may beconfigured to the UE. The UE may receive a UL grant for the UL CC 0through a PDCCH of the DL CC 0. In addition, a UL grant for the UL CC 1may be received through an E-PDCCH of the DL CC 1. In this case,according to the embodiment 4-1, the UE can implicitly know thatACK/NACK for a PUSCH transmitted through a UL CC 1 scheduled with thePDCCH must be transmitted through the PHICH and that ACK/NACK for aPUSCH transmitted through a UL CC 1 scheduled with the E-PDCCH must bereceived through the E-PHICH. In FIG. 15, the UL grants, the PHICH, andthe E-PHICH are indicated in the same subframe for convenience ofexplanation only, and thus they are not necessarily transmittedsimultaneously in the same subframe.

Embodiment 4-2: A BS may configure a UE to designate a PHICH monitoringcell and an E-PHICH monitoring cell by using an RRC message for eachcell in which a PUSCH is transmitted. That is, the BS may report aboutwhich channel is used between the PHICH and the E-PHICH to transmitACK/NACK for the PUSCH by using an RRC message for each subframe inwhich the PUSCH is transmitted. Alternatively, which channel is usedbetween the PHICH and the E-PHICH to transmit ACK/NACK for the PUSCH maybe configured by using an RRC message for each subframe. That is, the UEmonitors a corresponding channel according to a configured state.

FIG. 16 shows a method of receiving ACK/NACK of a UE according to theembodiment 4-2.

Referring to FIG. 16, DL CCs 0 and 1 and UL CCs 0 and 1 may beconfigured to the UE. A PHICH cell and E-PHICH cell for receivingACK/NACK may be indicated to the UE respectively for the UL CCs 0 and 1through an RRC message. For example, the RRC message may be used toreceive information indicating that ACK/NACK must be received through aPHICH of the DL CC 0 as to the UL CC 0 and that ACK/NACK must bereceived through an E-PHICH of the DL CC 1 as to the UL CC 1.

The UE receives UL grants for the UL CCs 0 and 1 through the E-PDCCH ofthe DL CC 0, and transmits a PUSCH in the UL CCs 0 and 1 according tothe UL grants. In addition, ACK/NACK for the PUSCH transmitted in the ULCC 0 is received through a PHICH of the DL CC 0, and ACK/NACK for aPUSCH transmitted in the UL CC 1 is received through an E-PHICH of theDL CC 1. In FIG. 16, the UL grants, the PHICH, and the E-PHICH areindicated in the same subframe for convenience of explanation only, andthus they are not necessarily transmitted simultaneously in the samesubframe.

Embodiment 4-3: A PHICH may be used in case of a UL HARQ processscheduled with a PDCCH, and an E-PHICH may be used in case of a UL HARQprocess scheduled with an E-PDCCH.

The aforementioned third and fourth embodiments may be used incombination.

A cell/subframe in which the PHICH or the E-PHICH is not transmitted maybe allowed not to be a monitoring cell of a UL grant. A cell in whichthe UL grant is located through either the PDCCH or the E-PDCCH is acell capable of transmitting ACK/NACK for a PUSCH, and it may beconfigured such that the UL grant and the PHICH (or E-PHICH) exist inthe same cell. This is for configuring a control signal related to ULscheduling in one cell having a good channel state.

In case of an extended carrier (or a new carrier type (NCT)) in which aPDCCH region is not configured, the UL grant may be transmitted throughan E-PDCCH. However, if the E-PHICH is not configured in the extendedcarrier, the UL grant may be subjected to cross-carrier scheduling froma cell in which the PHICH (or E-PHICH) is configured. A DL grant may besubjected to self-carrier scheduling, or the DL grant may be located inthe same cell as the UL grant to simplify the scheduling.

In case of a cell in which a subframe having an E-PDCCH and a subframenot having the E-PDCCH co-exist, a method of configuring the E-PHICH maydiffer according to a state. For example, in case of the subframe havingthe E-PDCCH, a start OFDM symbol position of the E-PHICH may be setidentical to the E-PDCCH, and in case of the subframe not having theE-PDCCH, a corresponding UL HARQ process may be allowed to performtransmission without the PHICH (embodiment 2-4) or may be allowed to usea PHICH of another cell (e.g., a primary cell).

FIG. 17 shows a structure of a BS and a UE according to an embodiment ofthe present invention.

A BS 100 includes a processor 110, a memory 120, and a radio frequency(RF) unit 130. The processor 110 implements the proposed functions,procedures, and/or methods. For example, the processor 110 may allocatea plurality of serving cells to a UE, and may transmit PHICH cellindicator information indicating a PHICH cell by using a higher layersignal. Further, the processor 110 determines whether to perform crosscarrier scheduling, and transmits a UL grant through a PDCCH or anE-PDCCH. In addition, the processor 110 receives a PUSCH from the UE,and transmits ACK/NACK for data included in the PUSCH through a PHICH oran E-PHICH. If a rule for a specific cell and channel through whichACK/NACK is transmitted is implicitly predetermined between the BS andthe UE, the PHICH cell indicator information may be unnecessary. Thememory 120 coupled to the processor 110 stores a variety of informationfor driving the processor 110. The RF unit 130 coupled to the processor110 transmits and/or receives a radio signal.

A UE 200 includes a processor 210, a memory 220, and an RF unit 230. Theprocessor 210 implements the proposed functions, procedures, and/ormethods. For example, the processor 210 may receive ACK/NACK for PUSCHtransmission through a PHICH or an E-PHICH according to the methoddescribed above with reference to FIG. 13 to FIG. 16. Optionally, theACK/NACK may be received by using a DCI format included in the PDCCH orthe E-PDCCH instead of an additional channel (i.e., PHICH or E-PHICH).The memory 220 coupled to the processor 210 stores a variety ofinformation for driving the processor 210. The RF unit 230 coupled tothe processor 210 transmits and/or receives a radio signal.

The processors 110 and 210 may include an application-specificintegrated circuit (ASIC), a separate chipset, a logic circuit, a dataprocessing unit, and/or a converter for mutually converting a basebandsignal and a radio signal. The memories 120 and 220 may include aread-only memory (ROM), a random access memory (RAM), a flash memory, amemory card, a storage medium, and/or other equivalent storage devices.The RF units 130 and 230 may include one or more antennas fortransmitting and/or receiving a radio signal. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememories 120 and 220 and may be performed by the processors 110 and 210.The memories 120 and 220 may be located inside or outside the processors110 and 210, and may be coupled to the processors 110 and 210 by usingvarious well-known means.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. Therefore, the scope of theinvention is defined not by the detailed description of the inventionbut by the appended claims, and all differences within the scope will beconstrued as being included in the present invention.

What is claimed is:
 1. A method for transmittingacknowledgement/not-acknowledgement (ACK/NACK) information to a userequipment (UE) to which a plurality of serving cells are allocated, themethod performed by a base station (BS) and comprising: receiving, fromthe UE, uplink data through a physical uplink shared channel (PUSCH) ofa first serving cell; and transmitting, to the UE, ACK/NACK informationfor the uplink data through a physical hybrid-ARQ indicator channel(PHICH), wherein if the BS configures the UE to receive an uplink grantwhich schedules the PUSCH through an enhanced physical downlink controlchannel (EPDCCH) of a second serving cell, the BS transmits the ACK/NACKinformation through a PHICH of the second serving cell, and wherein theEPDCCH is a control channel which is located in a data region.
 2. Themethod of claim 1, wherein the second serving cell is different from thefirst serving cell.
 3. The method of claim 1, wherein the EPDCCHcomprises only a UE-specific search space.
 4. The method of claim 1,wherein the data region comprises resources where a physical downlinkshared channel (PDSCH) can be allocated.
 5. A base station (BS)comprising: a transceiver for transmitting and receiving a radio signal;and a processor operatively coupled to the transceiver, wherein theprocessor is configured to: receiving, from the UE, uplink data througha physical uplink shared channel (PUSCH) of a first serving cell; andtransmitting, to the UE, ACK/NACK information for the uplink datathrough a physical hybrid-ARQ indicator channel (PHICH), wherein if theBS configures the UE to receive an uplink grant which schedules thePUSCH through an enhanced physical downlink control channel (EPDCCH) ofa second serving cell, the BS transmits the ACK/NACK information througha PHICH of the second serving cell, and wherein the EPDCCH is a controlchannel which is located in a data region.
 6. The BS of claim 5, whereinthe second serving cell is different from the first serving cell.
 7. TheBS of claim 5, wherein the EPDCCH comprises only a UE-specific searchspace.
 8. The BS of claim 5, wherein the data region comprises resourceswhere a physical downlink shared channel (PDSCH) can be allocated.